I have a GD 2003 WRX sti and I have just taken my roof racks off this weekend.
They are held on by a piece of metal that slides under the black stripe. So to take them off un bolt the racks off and then you have the mounts left, then take them off and you have the wiggle the metal bit out of/ under the black stripe.
Easy as pie.....its how it works on the 2003 wrx, should be the same as the GC series